Cisco recommends a maximum of one NM-1HSSI in a 2691, 2800,3725, or a 3825 and a maximum of two NM-1HSSIs in a 3745 or 3845.

The High Speed Serial Interface (HSSI) is a single port, network module
interface that supports speeds up to 52 Mbps over a Wide Area Network.
The HSSI network module for the 3600 requires an external T3/E3 CSU for
connectivity to the WAN, or can be used in a back-to-back configuration,
without the requirement for an external CSU. For use in the Enterprise
or Service Provider environment; the HSSI network module can be used for
point-to-point connections, Frame-Relay services, or used with inverse
multiplexers to distribute your bandwidth over multiple T1/E1 lines.Figure 18-142: High-Speed Serial Interface for the 3600